PROJECT SUMMARY This proposal uses innovative approaches to define an underlying molecular mechanism of the second most common form of young onset dementia, frontal temporal dementia (FTD). There are no disease-modifying treatment options available for FTD patients. A mutation in C9ORF72 (C9) is the most common cause of FTD. This mutation leads to an abnormal excess of DNA structures, termed G-quadruplexes. Excess G-quadruplex structures leads to the accumulation of toxic RNA and protein species and disrupts the transfer of materials between the cytoplasm and nucleus. The net effect of these toxicities is neuronal degeneration and FTD onset. Thus, C9 FTD is foundationally a ?G-quadruplex disease?. Within the cell reside helicase enzymes that are capable of unwinding G-quadruplexes. These enzymes have the potential to exacerbate C9 FTD by facilitating the production of toxic RNA and protein species. This proposal seeks to determine the role of the major G- quadruplex helicase, DHX36 (aliases: G4R1 and RHAU), in C9 FTD. DHX36 unwinds DNA and RNA G- quadruplexes, modulates cellular stress response, and our preliminary data suggest a role for DHX36 in nucleocytoplasmic transport. We hypothesize that reducing DHX36 levels will decrease the abundance of toxic RNA and protein species and thus ameliorate C9 FTD disease. Similarly, we hypothesize that increasing DHX36 levels will increase the abundance of toxic RNA and protein and thus exacerbate the disease. We further hypothesize that DHX36 modulates nuclear envelope structure and nucleocytoplasmic transport. We will test these hypotheses using novel human neuronal cell lines and Dhx36-mutant-C9 FTD mouse models. Completion of this work will potentially identify DHX36 as a novel therapeutic target affecting three of the major pathological hallmarks of C9 FTD: toxic RNA foci, toxic dipeptide proteins, and aberrant nucleocytoplasmic transport. This proposal builds on an existing collaboration between 4 research groups: Smaldino et al. (Ball State University), Wang et al. (University of Virginia), Todd et al. (University of Michigan), and Vaughn et al. (Nanomedica. Inc.). Each research group brings a complimentary set of expertise to pursue C9 FTD research. Undergraduate and graduate students will be integrated at every stage in this project allowing them to gain authentic experience with innovative technologies applied to a human disease, for which there is a striking absence of effective treatments. This research presents a unique opportunity for students to make significant and impactful contributions to this field.